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After lengthy consideration, a decision analysis approach that focuses on the potential health benefits of vaccine candidates but also identifies cost considerations was selected as the most appropriate for the committee’s purpose. It provides insights on both the expected health benefits from a vaccine (i.e., the morbidity and mortality it could avert) and the costs of achieving those benefits.

Every method has limitations and drawbacks, and the proposed approach is no exception. It is important to note that some factors cannot be quantified and incorporated into such an analysis. Chapter 8 deals with issues of this kind that should be considered in the ultimate selection of vaccines for accelerated development.

ISSUES IN PROJECT RANKING METHODOLOGIES

Sources of Estimates

Data from case reports, published studies, government statistics, and other sources, as well as the subjective judgments of experts, are required for all of the methods described above. Expert judgments may be elicited either informally or by such formal procedures as the Delphi method (Dalkey, 1969). These are described further in Volume 1 of the committee’s report (Institute of Medicine, 1985).

Sequential or “Lexicographic” Methods

Sometimes ranking schemes are based on sequential rather than simultaneous consideration of objectives. One variation of this approach is often called “lexicographic” because, like the ordering of words in a dictionary, it first groups the candidates according to their performances on a selected attribute (e.g., number of deaths due to the disease), then according to a second attribute, and so forth, until all ties are broken. Obviously, the order in which the attributes are considered is important. The assumption inherent in such methods is that one does not need to look at any but the first attribute, except in the case of ties. Most decision analysts discredit the use of lexicographic methods (Keeney and Raiffa, 1976) , although sequential screening methods are sometimes necessary if the number of candidates is very large.

Interdependence Among Projects

The methods described above assume, in general, that the consequences of implementing one project are independent of which other projects also are selected. This may not be a valid assumption if, for example, costs of administration can be shared (e.g., the combination diphtheria, tetanus, pertussis vaccine), or if immunologic responses are related, either synergistically or antagonistically. If the assumption of independence does not hold, then the affected vaccines



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